Question

Atmospheric carbon dioxide dissolves in raindrops, resulting in a solution of carbonic acid that makes rain slightly acidic. Since carbonic acid is a diprotic acid, write the chemical equations that describe its stepwise ionization.

Short answer

The stepwise ionization of carbonic acid is as follows: 1st ionization: H2CO3(aq) -> HCO3-(aq) + H+(aq), 2nd ionization: HCO3-(aq) -> CO3^2-(aq) + H+(aq).

Step-by-step solution

Write the chemical formula of carbonic acid and its first ionization

The chemical formula for carbonic acid is H2CO3. For the first ionization step, one hydrogen ion (H+) is removed, and the remaining molecule becomes the hydrogen carbonate ion (HCO3-). The equation for the first ionization is: H2CO3(aq) -> HCO3-(aq) + H+(aq).

Write the chemical equation for the second ionization

The second ionization occurs when another hydrogen ion is removed from the hydrogen carbonate ion, resulting in the formation of the carbonate ion (CO3^2-). The equation for the second ionization is: HCO3-(aq) -> CO3^2-(aq) + H+(aq).

Key concepts

Diprotic Acid

When we talk about acids, one aspect that determines their behavior is the number of protons (hydrogen ions, H+) they can donate. A diprotic acid is an acid that can donate two protons to a base. This is important because the ability and tendency to donate more than one proton ultimately affects the strength and pH level of solutions they form.

Carbonic acid (H2CO3), as described in the exercise, is a perfect example of a diprotic acid. Upon dissolving in water, it can release one proton during the first stage of ionization and another during the second stage. Such two-step ionization makes the study of diprotic acids slightly more complex than monoprotic acids, which only release one proton.

Understanding diprotic acids is crucial in many fields, including geochemistry, where the acidification of rain plays an essential role in environmental processes, and in physiology, where carbonic acid maintains the pH balance in the human body.

Ionization Steps

Ionization of diprotic acids, like carbonic acid, occurs in steps. Each step represents a stage in which a proton is released.

The first ionization step involves the release of the first proton, resulting in the formation of a hydrogen carbonate ion (HCO3⁻ ). The remaining molecule, now a hydrogen carbonate ion, still has the potential to release one more proton. This first ionization can symbolize the initial response of the acid to the aqueous environment.

In the second ionization step, the hydrogen carbonate ion (HCO3⁻ ), which formed from the first ionization, further ionizes by releasing another proton, resulting in a carbonate ion (CO3²⁻ ). This two-stage process highlights the complexity and gradual nature of ionization for diprotic acids, influencing how we calculate the final pH of the solution and understand the acid's strength and buffering capacity.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions, detailing which reactants yield which products, alongside their phase and proportions. In the context of diprotic acid ionization, these equations provide a clear image of how each ionization step unfolds.

For carbonic acid, its ionization is represented by two main equations:
For the first ionization step:
\[H2CO3(aq) \rightarrow HCO3⁻(aq) + H⁺(aq)\]
This shows carbonic acid (H2CO3) in aqueous solution donating a proton to form hydrogen carbonate (HCO3⁻ ) and a free hydrogen ion (H⁺ ).

For the second ionization step:
\[HCO3⁻(aq) \rightarrow CO3²⁻(aq) + H⁺(aq)\]
This equation demonstrates the second proton donation, transforming the hydrogen carbonate ion into a carbonate ion (CO3²⁻) with the release of another hydrogen ion.

Understanding these equations is key for students not only to follow the progress of the reaction but also to predict the outcomes, such as the concentration of ions at equilibrium in the solution, which, in turn, influences the solution's acidity.